The present invention relates to a particulate sealant for forming plugs in selected cells of honeycomb structures for employment in diesel exhaust filtration. In particular the present invention relates to a non-aqueous powdered material comprising a ceramic blend and an organic binder system.
It is well known that a solid particulate filter body, such as a diesel particulate filter, may be formed of a structure comprising a matrix of intersecting, thin, porous walls which extend across and between two of its opposing end faces and form a large number of adjoining hollow passages or cells which also extend between and are open at the end faces of the structure. To form a filter, one end of each of the cells is closed, a first subset of cells being closed at one end face and the remaining cells being closed at the remaining opposing end face of the structure. Either of the end faces may be used as the inlet face of the resulting filter. The contaminated fluid is brought under pressure to the inlet face and enters the body via those cells which have an open end at the inlet face. Because these cells are closed at the outlet end face of the body, the contaminated fluid is forced through the thin, porous walls into adjoining cells which are sealed at the inlet face and open at the outlet face of the filter body. The solid particulate contaminant in the fluid which is too large to pass through the porous openings in the walls is left behind and a cleansed fluid exits the filter body through the outlet cell channels, for use.
Up to this time selected cells were sealed or plugged with a foam-type cement, as disclosed in U.S. Pat. No. 4,297,140. The cement is formed into a paste by mixing ceramic raw material with an aqueous binder, such as methyl cellulose, plasticizer and water. When using this foam-type cement, both ends of the honeycomb structure are covered with flexible masks having holes through which the cement is pushed into the ends of the cells. There are numerous disadvantages associated with this type of filling or plugging material. The masks must be cleaned and dried after each use. Unclean masks can cause missing plugs requiring additional manual labor. The cement batch is time, shear and temperature dependent; often thrown out, unused due to age restrictions; and, drying is required to remove the water content.
It would be desirable to obtain a nonaqueous sealant for forming plugs in honeycomb cells, which avoids the aforementioned disadvantages.
It has now been discovered that a material capable of forming plugs in selected cells of a honeycomb structure can be made from a mixture comprising a ceramic blend of controlled composition and a binder system. The inventive material can be generally characterized as a non-aqueous particulate sealant. By xe2x80x9cnon-aqueousxe2x80x9d is meant that a water phase is absent from the composition. By xe2x80x9cparticulatexe2x80x9d is meant a material in powder form.
The composition of the sealant material according to the present invention consists essentially, by weight, of about 70-90% ceramic blend, and about 10-30% non-aqueous binder, preferably about 78-84% ceramic blend, and about 16-22% binder. The ceramic blend is a mixture of ceramic raw ceramic materials which are selected to form a composition of MgO, Al2O3, and SiO2 that will yield on firing cordierite having a stoichiometry approximating Mg2Al4Si5O18, as disclosed in U.S. Pat. No. 5,258,150 assigned to the present assignee, and herein incorporated by reference in its entirety. The composition preferably consists essentially of, in percent by weight of about 12 to 16% MgO, about 33 to 38% Al2O3, and about 49 to 54% SiO2. The most preferred composition consists essentially of in percent by weight about 12.5 to 15.5% MgO, 33.5 to 37.5% Al2O3, and 49.5 to 53.5 SiO2. The actual raw materials for the MgO, Al2O3, and SiO2 composition components are talc having a BET surface area of no greater than about 5 m2/g, and preferably no greater than about 3 m2/g, clay, such as platelet or stacked clay, an aluminum oxide yielding component having an average particle size of about 3 to 8 micrometers, and free silica. A pore former or burnout agent may be optionally included in the ceramic batch. Suitable pore formers include for example graphite, cellulose, flour and the like.
The binder system has to be compatible with a very high ceramics loading (i.e., 80-90% by weight). Accordingly, a suitable binder system comprises a thermoplastic polymer capable of forming a reversible gel as taught in U.S. Pat. No. 5,602,197, co-assigned to the present assignee and herein incorporated by reference in its entirety. Specifically, in the present invention, the binder system is composed of a high molecular weight thermoplastic polymer serving as a gel-forming species, a wax serving as the solvent for the thermoplastic polymer, the wax having a low melting point, and being selected from fatty alcohol, fatty acid, fatty glycol, and fatty glyceride waxes, and optionally a dispersant serving as a functional additive. Preferred high molecular thermoplastic polymers are a tri-block styrene-ethylene/butylene-styrene copolymer or a butyl methacrylate/acrylic acid copolymer. The tri-block styrene-ethylene/butylene-styrene copolymer is commercially available under the trade-name Kraton(copyright) available from Kraton Polymer Company of Houston, Tex. The butyl methacrylate/acrylic acid copolymer is commercially available under the trade-name as Neocrylo(copyright) available from NeoResins of Wilmington, Mass. The use of appropriate dispersants allows for very high inorganic solids loadings, which loadings would be difficult to achieve without the use of any dispersants in the binder system. Also added dispersants can have a substantial effect on the rheology. In the present invention a suitable dispersant is commercially available under the trade name Solsperse(copyright) available from Avecia of Charlotte, N.C. The particularly preferred binder system has a formulation consisting essentially, in weight percent, of about 5-20% wax, 1-7% high molecular weight thermoplastic polymer, and 0-5% dispersant A more preferred formulation consists essentially, in weight percent, of about 9.8-10.0% wax, 4.9-5.0% high molecular weight thermoplastic polymer, and 1.7% dispersant.
Alternatively, the binder system comprises a solid grade thermosetting resin. See U.S. Pat. No. 5,043,369. Unlike thermoplastic polymers which on heating soften and flow, and on cooling re-solidify, thermosetting resins have chains that are linked in a 3-D network and cannot be melted, often getting stiffer with heating. Representative thermosetting resins suitable in the present invention include epoxy resins, phenolics, diallyl phthalates, unsaturated polyesters and functionalized acrylics. Unlike prior art batches for ceramic forming, which considered residual carbon, from a thermosetting resin, remaining after binder removal, to be detrimental to the development of desirable ceramic microstructure in the final product, the sealant material of the present invention is not so sensitive to residual carbon remaining after debinding. Since the sealant material is used to form plugs in diesel particulate filters, the porosity that would be produced by residual carbon would not be harmful. A preferred binder system formulation comprising a thermosetting resin consists essentially of in percent by weight about 20-30% thermosetting resin, and 0-2% dispersant. A preferred thermosetting rein is epoxy resin such as Epon(copyright) available commercially from Resolution Performance Products of Houston, Tex. Optionally the binder system could include a crosslinking agent as known in the art, such as polyamines, phenolics, amino resin or dibasic acid.
In the process of making the inventive sealant, the preferred method comprises preparing an intimate mixture of the ceramic blend and the binder through high shear dispersive mixing according to the teachings of U.S. Pat. No. 5,043,369, co-assigned to the present assignee and herein incorporated by reference in its entirety. First the ceramic raw materials are pre-mixed to form a homogeneous ceramic blend. Thereafter, the ceramic blend is dry mixed with the wax, polymer, and dispersant in accordance with conventional procedures. The resulting batch is fed into a twin screw extruder which can be programmed to operate at temperatures between about 30xc2x0-140xc2x0 C.; temperatures at which the binder fluidizes and very fine melt mixing can occur to form a homogeneous paste. Thereafter, the so-mixed material is extruded in a spaghetti-like form, cooled and granulated for later use as a sealant.
Alternatively the inventive sealant material may be compounded according to the teachings of U.S. Pat. No. 5,602,197, by simply combining the selected ceramic powder material with premixed binder in accordance with conventional procedures for using hot melt binders. In one embodiment the ceramic blend is first combined with the dispersant component and a solvent for the dispersant to provide a powder slurry. In a separate container and separate mixing step, the thermoplastic polymer selected for incorporation in the binder is combined with a selected low-melting wax component at a temperature above the melting point of the wax, in order to provide a wax/polymer mixture comprising a uniform solution or dispersion of the polymer in the molten wax. The powder slurry is next combined with the wax/polymer mixture and the combination is mixed together at a temperature above the melting temperature of the wax. Mixing is continued for a time at least sufficient to provide a homogeneous dispersion of the powder in the binder, and will be sufficient to evaporate the solvent component from the mixture. Thereafter, the batch is removed, cooled, and granulated for later use in plugging honeycomb cells.
Alternatively, individual, spherical granules comprising the sealant may be formed by art known spray drying techniques, from a well dispersed slurry source containing the ceramic blend and the polymeric binder system.
Regardless of forming methods however, the sealant is formed into powder form having a mean particle size of between about 5 and 500 micrometers, preferably 25-250 micrometers, a range of size which allows for good flowing and packing capabilities.